1. Field of the Invention
This invention relates generally to hydraulic cylinders, and pertains more particularly to a hydraulic cylinder provided with corrugated running or bearing surfaces.
2. Description of the Prior Art
The manufacturers of hydraulic cylinders strive to achieve uniform clearances. However, in practice, the diameters of the pistons will vary with respect to each other, and the diameters of the casing bores will vary with each other. If a piston having a diameter at one end of the tolerance range is placed in a bore having a diameter at the other end of the tolerance range, then it is apparent that the worst type of clearance condition exists. In addition, there can be ovality differences between bores. All of which factors contribute to having running fits that are too loose, too tight and non-uniform. Therefore, these shortcomings result in non-uniform performance and service life.
Hydraulic cylinders make use of theoretically cylindrical running or bearing surfaces. In practice, these "cylindrical" surfaces are not truly cylindrical, either being oval to begin with or becoming so in use. The degree of ovality varies quite widely because of a number of factors.
Most piston-type hydraulic cylinders are double-acting which means that oil under pressure is selectively introduced into either end of the casing to provide movement in the proper direction. Usually, one end of the casing is "closed" and the other "open", the piston rod extending through the open end. In such a situation, the piston in conjunction with the bore provides one running or bearing surface and the journal provided in the cylinder head at the open end of the casing in conjunction with the piston rod provides another running or bearing surface.
In an effort to lessen troubles experienced in the field, the factory has at times resorted to selecting a piston found to have one diameter and selecting a casing have a suitable bore diameter for the piston that has been selected in order to obtain the optimum clearance between these two components. This practice permits greater variations in the tolerances during the making of the parts, and the effect of the otherwise larger tolerances can be reduced via the selection route. However, matching a given piston with a particular casing takes time and is a costly procedure. The problem of bore ovality, however, is not overcome by resort to this select fit process.
In a number of cases, expensive bearing metal overlays have been applied to the piston bearing surface and/or the head journal for the piston rod. Another attempt to solve the problem has been to employ bearing wear rings on the piston surfaces. Neither of the above contribute to providing optimum clearances and conformability, particularly in the case of the alluded to wear rings. Actually, the wear ring procedure aggravates the non-uniformity of diameter fit problem, since it adds to component tolerance stacks and materially reduces soft seal life and since it requires additional clearance between the piston and the casing bore. This increased clearance objectionably increases the so-called extrusion gap. More specifically, there is a reduction in the support at each side of the soft seal.
With respect to the journals located in the cylinder heads of hydraulic cylinders, a problem arises in this region regarding improper clearances. In this regard, the journal is smaller than the piston diameter. Consequently, the journal is subjected to higher side load hot spots and wear than the piston, because the unit loading is normally appreciably greater. Owing to the reduced amount of contact, this being because of the smaller diameter, the more concentrated loading produces the hot spots because of the lack of an adequate oil film. Additionally, where there is a misalignment or lack of concentricity between the piston rod and casing bore, this misalignment is also present as far as the cylinder head journal and piston rod. The misalignment further contributes to the presence of hot spots.